Method for forming an electrically conducting mark on a non-conducting surface



United States Patent METHOD FOR FORMING AN ELECTRICALLY CONDUCTING MARKON A NON-CONDUCTING SURFACE Oliver A. Short, Metuchen, NJ assignor to E.I. du Pont de Nemours and Company, Wilmington, Del., a corporation ofDelaware N0 Drawing. Filed July 7, 1958, Ser. No. 746,627

4 Claims. (Cl. 117212) This invention relates to a new and improvedprocess for positioning an electrically conducting mark on anonconducting surface. In its most practical application the inventionrelates to the application of electrically conducting marks on papersurfaces.

Many attempts have been made heretofore to provide a satisfactory methodof writing, printing, or otherwise applying an electrically conductivemark on paper or similar non-conductive surfaces. Writing or printingwith graphite or metallic inks or pencils on paper has not beenheretofore satisfactory. Either such written or printed marks were notsufficiently conductive, or they did not adhere sufiiciently well to thepaper.

It is, therefore, an object of this invention to provide an improvedmethod for the application of electrically conducting marks on paper.

It is a further object to provide a permanent, adherent, highlyelectrically conductive mark on paper or like nonconductive surface.

Other objects of the invention will become apparent by the descriptionthat follows.

These objects may be accomplished by coating or impregnating paper withan aqueous solution of a hydrazine sulfate, drying the same andsubsequently writing, printing or otherwise applying marks or indicia ofany kind on said dry paper with an aqueous ammoniacal complex silversalt solution. Even though the dry paper containing a hydrazine sulfateis exposed to atmospheric conditions for weeks, the ammoniacal silversalt solution will react with the dry hydrazine sulfate to produce acontinuous film of metallic silver which is highly electricallyconductive.

The hydrazine sulfate may be monohydrazine sulfate, N H -H SOdihydrazine sulfate (N H -I-I SO or hydrazine disulfate, N H -2H SO Allthree substances are solids that can be characterized by their physicaland crystallographic properties. On the basis of its greater solubilityin water, the dihydrazine sulfate is preferred. At least 1% of thehydrazine sulfate is dissolved in water and the paper coated with asufficient quantity thereof to at least partially impregnate the paper.The paper is then dried, preferably at atmospheric temperature afterwhich it is ready to be written or printed on with the ammoniacal silversalt solution. The complex silver salt may be ammoniacal silver chloridenitrate or sulfate, preferably ammoniacal silver nitrate.

The aqueous ammoniacal silver salt solution may contain between 5% and75% silver nitrate. The solution may be contained in a fountain pen,ball-point pen, or on an ink pad for printing purposes. A line drawn orprinted on the hydrazine sulfate-containing surface of the paper willoperate to precipitate metallic silver in a continuous electricallyconductive mark.

The following examples are given to illustrate, in detail, the processof this invention.

Example I A piece of paper is coated with a 20% water solution ofhydrazine sulfate and allowed to dry at room tem- 3,032,443 Patented May1, 1962 ICC perature or at a slightly elevated temperature but below thedecomposition point of the reducing agent. An ink is made by dissolving50 grams silver nitrate in 50 cc.

water and adding concentrated ammonia water until the 5 precipitatewhich is first formed dissolves. An excess of about 5 cc. of ammonia isthen added. When the ink is applied to the paper by means of a pen, aclear watery image is first obtained but in less than 2 seconds theimage darkens and then becomes very black. This black image issutiiciently conducting to actuate electronic reading devices, i.e. lessthan 50,000 ohms for inch. Frequently resistances as low as ohms forinch are obtained.

Example II Example I was repeated using concentrations of hydrazinesulfate varying from 1% to 50%. Metallic silver marks produced on thedry hydrazine sulfate-containing paper had adequate conductivity toactuate electronic reading devices. The silver in marks on paperprepared with over 35% hydrazine sulfate had a tendency to spread butthe conductivity was satisfactory.

Example III The process of Example I was repeated using ammoniacalsilver chloride instead of silver nitrate. Also the concentration of theammoniacal silver nitrate solutions was varied from 5% to 75%.

In all cases the conductivity of the lines or marks produced wasadequate for use in mark sensing devices.

Throughout the specification and claims percentages, parts andproportions refer to percentages, parts and proportions by weight unlessotherwise specified.

Since it is obvious that many changes and modifications can be made inthe above-described details without departing from the nature and spiritof the invention, it is to be understood that this invention is not tobe limited to said details except as set forth in the appended claims.

I claim:

40 l. The method of forming an electrically conducting mark on paperwhich comprises applying on said paper surface an aqueous solution of ahydrazine sulfate, drying said surface, and marking on said surface bymarking thereon with an aqueous ammoniacal silver salt solutionsubstantially at room temperature.

2. The method of claim 1 in which the ammoniacal silver salt isammoniacal silver nitrate.

3. The method of claim 1 in which the aqueous so lution of a hydrazinesulfate has a concentration be- 50 tween 1% and 50% by weight.

4. The method of claim 1 in which the ammoniacal silver salt solution isan ammoniacal silver nitrate solution having a concentration of 5% to75% by weight.

Wein: (H) Metallizing Non-Conductors, 1945, Metal Industry PublishingCo., New York, N.Y., page 27 relied on.

Wein: Hydrazine-Its Use in Mirror Making, The Glass Industry, August1955, pp. 413-416 and 422.

1. THE METHOD OF FORMING AN ELECTRICTALLY CONDUCTING MARK ON PAPER WHICHCOMPRISES APPLYING ON SAID PAPER SURFACE AN AQUEOUS SOLUTION OF AHYDRAZINE SULFATE, DRYING SAID SURFACE, AND MARKING ON SAID SURFACE BYMARKING THEREON WITH AN AQUEOUS AMMONIACAL SILVER SALT SOLUTIONSUBSTANTIALLY AT ROOM TEMPERATURE.